How to use this calculator
Pick whether you want to enter your numbers in imperial (feet and pounds) or metric (centimeters and kilograms) using the toggle at the top. Fill in your height and weight. Hit calculate and you will instantly see your BMI, your WHO category, where you fall on the BMI range, your BMI Prime ratio, and your healthy weight range for that height.
The Advanced tab adds optional fields for age and sex. The BMI number itself does not change — those fields are used only to add context to the interpretation, since BMI carries different meaning for older adults and during the teenage years.
What is BMI?
Body Mass Index is a single number that describes the ratio of your weight to your height. It was originally developed by Belgian mathematician Adolphe Quetelet in the 1830s as a way to study the average build of populations. Physiologist Ancel Keys renamed it Body Mass Index in 1972 and it has been the standard screening tool for body weight ever since.
BMI is popular for one reason: it requires only two measurements that almost anyone can take at home. It does not need calipers, scans or specialized equipment. That convenience is also its main limitation — it does not know whether your weight is muscle, fat, bone or fluid.
BMI categories and what they mean
The World Health Organization defines five broad BMI categories for adults. The CDC uses the same cutoffs. A reading inside the healthy range is associated with the lowest risk of weight-related disease in most large population studies.
| BMI Range | Category | What it means |
|---|---|---|
| Below 18.5 | Underweight | Below healthy weight range |
| 18.5 — 24.9 | Normal weight | Healthy weight range |
| 25.0 — 29.9 | Overweight | Above healthy weight range |
| 30.0 — 34.9 | Obese, class I | Moderate |
| 35.0 — 39.9 | Obese, class II | Severe |
| 40.0 and above | Obese, class III | Very severe |
Source: BMI categories from the World Health Organization — Obesity and Overweight Fact Sheet and the CDC Adult BMI guidelines. Class I, II, III obesity subcategories follow WHO classification.
How is BMI calculated?
The formula is straightforward. Weight in kilograms divided by height in meters squared. The imperial version uses a conversion factor of 703 so that weight in pounds and height in inches produce the same final number.
BMI = Weight (kg) ÷ Height (m)²
// Imperial — weight in lbs, height in inches
BMI = 703 × Weight (lbs) ÷ Height (in)²
// BMI Prime — your BMI divided by the healthy ceiling
BMI Prime = BMI ÷ 25
Worked example. A person who is 5 feet 9 inches tall (175 cm) and weighs 160 lbs (72.6 kg) has a BMI of 160 × 703 ÷ 69² = 23.6. That falls comfortably inside the healthy range of 18.5 to 24.9.
What is a healthy BMI?
For adults aged 20 to 64, a BMI between 18.5 and 24.9 is considered healthy. The middle of that range — roughly 21 to 22 — is the value most often used as a practical target. There is no medical advantage to landing exactly at 22 versus 23, but pointing at the middle of the range gives most people some room to drift in either direction without crossing a category line.
Your healthy weight range is the span of weights that put you between BMI 18.5 and 24.9 at your specific height. The calculator above shows this range for you whenever you enter a valid height. For example, a 5 foot 9 inch adult has a healthy weight range of roughly 125 to 169 lbs (57 to 77 kg).
BMI Prime — a useful companion metric
BMI Prime is your BMI divided by 25, the upper cutoff of the healthy range. It turns the BMI number into a ratio that is easier to read at a glance.
- BMI Prime under 0.74 — underweight (your BMI is below 18.5).
- BMI Prime between 0.74 and 1.00 — healthy.
- BMI Prime between 1.00 and 1.20 — overweight (you are up to 20% above the healthy ceiling).
- BMI Prime above 1.20 — obese.
The benefit of BMI Prime is that it gives you a quick sense of how far you are from the healthy ceiling, not just whether you are above or below it.
Is BMI accurate for everyone?
BMI is a useful screening tool but it is not a diagnosis. The categories are based on population averages and there are well-known groups where BMI either overstates or understates body fat.
- Muscular athletes — strength athletes and bodybuilders often have BMIs in the overweight or obese range while carrying very low body fat. Their weight is muscle, not fat.
- Older adults — people over 65 may lose muscle and bone density while gaining fat, which can mask a high body fat percentage at a normal BMI.
- People with very low muscle mass — a sedentary person can have a normal BMI while carrying high body fat, sometimes called "skinny fat."
- Pregnancy and postpartum — BMI does not apply during pregnancy or in the early postpartum period.
- Children and teenagers — BMI is interpreted using age-and-sex percentile charts, not the adult categories.
Important
Not medical advice. BMI is a screening tool and does not diagnose health, body composition or fitness. This calculator is intended for adults aged 20 and older. If your result concerns you, talk to your clinician — they can interpret it alongside body composition, family history, blood work and other risk factors.
- Children and teens: BMI is interpreted using BMI-for-age percentiles. Use the CDC growth charts or ask your pediatrician.
- Pregnancy and postpartum: BMI does not apply during these periods.
- If you have or suspect an eating disorder, please seek professional support before making weight change plans.
BMI in adults 65 and older
Several large studies suggest the BMI range associated with the lowest mortality risk shifts slightly higher in older adults. A 2014 meta-analysis covering more than 200,000 older adults found the lowest risk of death between BMI 23 and 27. Being underweight (BMI under 23) in older adulthood carried higher risk than being mildly overweight, largely because of muscle loss, bone density loss and frailty.
The takeaway: a 70-year-old with a BMI of 26 is not in the same risk bucket as a 30-year-old with the same number. The categories still apply, but the lower edge of "healthy" deserves more attention than the upper edge for seniors.
Source: Winter, J. E. et al. (2014). BMI and all-cause mortality in older adults: a meta-analysis. The American Journal of Clinical Nutrition, 99(4), 875–890.
BMI vs body fat percentage vs Body Roundness Index
BMI is one of several ways to estimate body composition. Each one uses different inputs and answers a slightly different question.
- BMI — height and weight only. Best for: a fast population-level screen.
- Body fat percentage — measures actual fat tissue using calipers, the US Navy circumference method, bioimpedance scales or DEXA. Best for: people with high or low muscle mass where BMI misleads.
- Body Roundness Index (BRI) — uses waist circumference and height. A 2024 JAMA study found BRI predicted all-cause mortality more accurately than BMI for many people. Best for: assessing central (abdominal) fat.
- Waist-to-height ratio — a simpler abdominal-fat proxy. Healthy is usually under 0.5.
If your BMI puts you near a category boundary, layering in body fat percentage or BRI usually clarifies the picture more than recalculating your BMI a different way.
Frequently asked questions
For most adults, a BMI between 18.5 and 24.9 is considered healthy by the World Health Organization. A BMI of 21 to 22 sits in the middle of that range and is a common practical target.
BMI equals weight in kilograms divided by height in meters squared. In imperial units, BMI equals 703 times weight in pounds divided by height in inches squared. The result is the same number regardless of which unit system you use.
BMI does not distinguish between fat and muscle. A muscular athlete can have a BMI in the overweight or obese range while carrying very little body fat. For people with above-average muscle mass, body fat percentage or the Body Roundness Index is a more accurate measure of body composition than BMI alone.
No. For people under 20, BMI is interpreted using age and sex specific percentile charts rather than the adult categories. A clinician compares the child's BMI to a CDC growth chart to determine their percentile.
BMI Prime is your BMI divided by 25, which is the upper limit of the healthy range. A BMI Prime under 1.0 means you are within the healthy range. A BMI Prime of 1.2 means you are 20 percent above the healthy ceiling. It is a quick way to see how far you are from the cutoff.
The same BMI formula and the same WHO category thresholds apply to both men and women. Women on average carry more body fat than men at the same BMI, but the standard categories remain the most widely used reference for adults of either sex.
Some research suggests a slightly higher BMI of 23 to 27 is associated with the lowest mortality in adults over 65. Being underweight in older adulthood carries health risks because of muscle loss and bone density loss. The WHO categories still apply but the lower end of healthy should be treated cautiously.
For most adults, checking BMI once every few months is enough. BMI changes slowly because both height and weight change slowly. If you are actively losing or gaining weight, monthly checks are useful to track trends.
There is no difference in the result. Metric uses kilograms and meters in the formula directly. Imperial uses pounds and inches with a 703 multiplier to convert. Both produce the same BMI number. Our calculator accepts either unit and converts automatically.
BMI is calculated from height and weight only and does not measure fat directly. Body fat percentage measures the actual proportion of fat tissue in your body using methods like skinfold calipers, DEXA scans or the US Navy circumference formula. Two people with the same BMI can have very different body fat percentages.